PRESSURE-TEMPERATURE DEPENDANT COMPOSITIONAL VARIATIONS IN AMPHIBOLES IN MAFIC ROCKS

Amphiboles encompass a wide range of compositional variability. In high variance mafic assemblages, changes in amphibole chemistry document changes in P-T conditions. In order to gain a solid understanding of the metamorphism of mafic rocks under widely varying P-T conditions, an understanding of amphibole compositional variability, and the potential exchange mechanisms operating under differing P-T regimes, is essential. For many years, workers have relied on seminal studies by workers such as Lard and Albee (1981 a and b). These authors, however, only compared compositions for medium P-T series rocks through staurolite grade, and high P-T series blueschist facies rocks. Such comparisons for higher temperature amphibolite to granulite facies rocks metamorphosed under low-to-high pressure conditions have not been made in the past. In order to better understand variations in amphibole compositions under high-grade conditions, we have collected P-T and compositional data from numerous published sources. Both experimental and natural data are represented. P-T compositional variability was assessed by bracketing amphibole compositions equilibrated under known pressures and temperatures. These data define pressure related trends consistent with coupled Na-Al substitutions which are in turn consistent with pressure related pargasite exchange. Significantly, these data are consistent with trends noted by Colberg (2001) for 4-14 kbar, nearly isothermal, mafic assemblages reported in the Carolina Terrane. Also significant are consistent breaks in trends for the plotted data in the range of 5-7 kbar. This is consistent with the position of epidote controlling equilibria calculated by Colberg (2001).